The kidneys play a major role in degrading peptide hormones. Since circulating peptides influence a host of vital functions, it is imperative to define more completely the hydrolytic processes. An understanding of these mechanisms would provide: 1. a basis for synthesizing analogs resistant to hydrolysis; 2. an explanation for pathological conditions which may accompany alteration of the renal handling of toxic peptides; 3. a clearer picture of the kidney's role in conserving amino acids, and its influence upon the circulating levels of peptides. Recent work in our laboratory has indicated that certain small, linear peptides are hydrolyzed by enzymes in the renal brush border of the proximal tubule, followed by reabsorption of metabolites. The present proposal is intended to investigate the effects of altering size, composition, and structure of peptide molecules on the pattern of transport and hydrolysis. Specifically, labeled luteinizing hormone releasing hormone (LHRH), glucagon and insulin will be microperfused in separate experiments through rabbit renal tubular segments in vitro. Reabsorption of labeled material will be quantified, and products of hydrolysis or intact hormone appearing in the bathing medium and collection fluid will be studied by high voltage paper electrophoresis or descending paper chromatography. Analogs of LHRH will be similarly microperfused to examine the alterations of renal handling brought about by structural and compositional changes in the molecule. To more clearly define the degradative process, and identify the brush border as a site for enzymatic hydrolysis, the labeled peptide hormones will be incubated directly in the presence of microvilli membranes from rabbit brush border. Cleavage products will be characterized electrophoretically or chromatographically under varying conditions of temperature, time of incubation, and pH of the reaction mixture.